1. Field of the Invention
This invention relates to a novel aqueous drilling fluid and a method for drilling wells through subsurface formations by means of rotary drilling tools utilizing said fluid, and particularly to an aqueous drilling fluid having low solids content and high filtration rate for improved drilling rate and having shale controlling characteristics.
2. Description of the Prior Art
Drilling fluids, or drilling muds as they are sometimes called, are slurries of clayey solids used in the drilling of wells in the earth, such as are drilled for the purpose of tapping subterranean deposits of petroleum, gas and other fluid materials. Such fluids have a number of different functions, the most important of which involve removing cuttings from the well, sealing off permeable formations of gas, oil, or water which may be encountered at various levels as the well is drilled into the subterranean formations, lubricating the drilling tool and drill pipe which carries the tool, and holding the cuttings in suspension in the event of shutdown in drilling and pumping of the drilling fluid.
An ideal drilling fluid is a thixotropic fluid, i.e., a fluid whose apparent viscosity decreases as the degree of agitation or shear rate increases (such as is caused by pumping or otherwise circulating the fluid through the drill string); but when such agitation or shearing or circulation is halted, the fluid gels or forms a gel structure which will support the drilled cuttings to prevent them from falling back down into the bottom of the hole. The rate of gel formation must be such as to permit the cuttings to fall only a short distance before the gel structure is sufficiently strong to support them. It is important to maintain the degree of gellation and the rate of gellation within narrow limits, since excessive gel formation will be detrimental to resumption of the drilling operation, and insufficient gel formation will permit the formation cuttings to fall to the bottom of the hole, which can result in sticking of the drill pipe.
In modern rotary drilling technology, conventional practice involves the pumping of a drilling fluid having the proper viscosity, gel rate and gel strength down the center conducting passage of the drill string, through the jets in the drill bit attached to the bottom of the drill string, where the fluid jets downward and agitates the formation being drilled by the drill bit. The jetting action assists in the drilling operation, accumulates formation cuttings, and assists in transporting these cuttings away from the immediate area being drilled by the drill bit and up to the surface. The drilling fluid passes upward via the annular space between the exterior of the drill string and the wall of the hole being drilled. The drilling fluid must have sufficient viscosity so that during the time it is being pumped and otherwise agitated, it will support and transport the sand and formation cuttings in the return trip to the surface of the earth. In the event drilling operations and drilling fluid pumping are suspended, the gel rate and gel strength must be adequate to support the drilled solids and other particulate matter in the annular space, to prevent their falling to the bottom of the hole.
When the well is being drilled through highly permeable formations, materials are added to the drilling fluid to increase the tendency to form a low permeability mud cake on the borehole wall against the porous formation, as the liquid filtrate phase filters into the porous formation. In very permeable formations, it is desirable to have adequate colloidal material present in the drilling fluid to form a thin impermeable cake as quickly as possible. Rapid formation of the proper filter cake is necessary to reduce the filtration rate quickly as the cake forms, and also to avoid the development of an excessively thick cake which can increase the friction between the rotating drill string and the wellbore, and which can also reduce the annular flow passage by which the drilling fluid returns to the surface of the earth.
In some areas of the country, formations known as heaving or sloughing shales are penetrated as the well is drilled into the subterranean formations. As the well is drilled into these heaving shale formations when conventional aqueous or water-base drilling fluids are being employed in the drilling operation, considerable difficulty can be encountered. Certain shales, such as are encountered in the Gulf Coast area of Texas and Louisiana, contain considerable concentrations of mud making clays or minerals such as sodium montmorillonite which tend to swell upon hydration or absorption of water from the drilling fluid, and bring about an immediate increase in the viscosity and gel of the drilling fluid used. This addition of hydrated clay solids to the drilling fluids must be counteracted by watering or a chemical drilling fluid system must be employed to stabilize these heaving shale materials. In the case of heaving shale formations containing water hydratable clayey solids, drilling fluid systems have been developed which adequately stabilize the shale sections as they are being drilled. For example, U.S. Pat. No. 2,802,783 (1957), Weiss, et al. and U.S. Pat. No. 2,896,915 (1959) Weiss et al, describe chemicalized systems which have been extremely useful in drilling the type of mud making heaving shales encountered in the Gulf Coast region.
Another type of troublesome heaving or sloughing shale having similar external appearances but significant chemical differences from the above described Gulf Coast heaving shale sections, has been encountered in drilling in the Delaware Basin area of West Texas and New Mexico. This shale section, referred to as the Wolfcamp-Pennsylvanian-Mississippian interval of the Delaware Basin, is a predominately shale interval which, unlike the Gulf Coast shale sections, contains essentially no bentonite clay. The Wolfcamp shale is predominantly illitic in composition. It is believed that the heaving occurs because the shale has small fissures or cracks in it, which permit invasion by the drilling fluid or filtrate of the drilling fluid to cause hydration of the hydratable materials present in the shale, resulting in an unstable condition.
Historically, the Wolfcamp shale interval has been drilled by resorting to an aqueous drilling fluid which has been treated with colloidal materials to produce a very low filtration rate, in the range of 15 to 20 cc's when measured by the standard A.P.I. testing procedure. Although the Wolfcamp shale interval is essentially impermeable, the presence of colloidal filtration control additives decreases the extent of entrance of the drilling fluid or filtrate thereof into the small fissures in the shale, and thereby decreases the sloughing tendency to some extent. Also, the drilling fluid density, or mud weight as it is referred to in drilling fluid technology, is maintained at a greater level than is necessary to produce a hydrostatic pressure sufficient to offset the pore pressure of the formation being drilled. Drilling fluid densities, or drilling mud weights, of 11 to 14 pounds per gallon have been employed in drilling this interval, which is in excess of the drilling fluid density necessary to produce a hydrostatic pressure in excess of the gas pressures encountered in this interval.
Materials used to achieve this filtration rate reduction are generally colloidal materials such as starch, carboxymethycellulose, or water yielding clays such as bentonite. These materials, together with the natural clays dispersed in the highly dispersive aqueous drilling fluid commonly employed in this area, result in a relatively high solids content, e.g., in the range of from 12 to 18 percent total solids for mud weights of only around 101/2 pounds per gallon.
Unfortunately, while the above described operating procedures have been moderately effective in controlling the sloughing tendency of the Wolfcamp shale interval, all of these corrective measures increase the cost of the drilling fluid being employed in drilling the well. Even more significantly, all of the factors described above, including reduced filtration rate, increased mud weight, and increased total solids content, cause a significant decrease in the drilling or penetration rate. Since the drilling rigs employed in the drilling of such wells are quite expensive, the increased time required to drill a well to a predetermined depth can have a more significant impact on the total cost of the well than the direct expenditures for drilling fluid chemicals. This will be shown more precisely in specific field examples contained later in this specification.
Some recognition of this specific problem is shown in the publication in the Oil and Gas Journal for May 29, 1972, "New mud holds shales, allows fast drilling in West Texas", by Mr. John L. Kennedy. The cited publication relates the experience of a West Texas operator in drilling wells in the Delaward Basin using a salt brine fluid containing a polysaccharide hydrophilic polymer which appears to stabilize the Wolfcamp shale interval to some extent.
It is a goal of the preferred embodiments of this invention to provide an improved drilling fluid which achieves significant stabilization of non-mud making shale formations such as the Wolfcamp formation, while still providing optimum properties for low cost, rapid drilling rate, and trouble-free operation.